Written by Mattt Thompson on

Here's a question: why isn't NSOrderedSet a subclass of NSSet?

It seems perfectly logical, after all, for NSOrderedSet--a class that enforces the same uniqueness constraint of NSSet--to be a subclass of NSSet. It has the same methods as NSSet, with the addition of some NSArray-style methods like objectAtIndex:. By all accounts, it would seem to perfectly satisfy the requirements of the Liskov substitution principle, that:

If S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program.

So why is NSOrderedSet a subclass of NSObject and not NSSet or even NSArray?

Mutable / Immutable Class Clusters

Class Clusters are a design pattern at the heart of the Foundation framework; the essence of Objective-C's simplicity in everyday use.

But class clusters offer simplicity at the expense of extensibility, which becomes especially tricky when it comes to mutable / immutable class pairs like NSSet / NSMutableSet.

As expertly demonstrated by Tom Dalling in this Stack Overflow question, the method -mutableCopy creates an inconsistency that is inherent to Objective-C's constraint on single inheritance.

To start, let's look at how -mutableCopy is supposed to work in a class cluster:

NSSet* immutable = [NSSet set];
NSMutableSet* mutable = [immutable mutableCopy];

[mutable isKindOfClass:[NSSet class]]; // YES
[mutable isKindOfClass:[NSMutableSet class]]; // YES

Now let's suppose that NSOrderedSet was indeed a subclass of NSSet:

// @interface NSOrderedSet : NSSet

NSOrderedSet* immutable = [NSOrderedSet orderedSet];
NSMutableOrderedSet* mutable = [immutable mutableCopy];

[mutable isKindOfClass:[NSSet class]]; // YES
[mutable isKindOfClass:[NSMutableSet class]]; // NO (!)

That's no good... since NSMutableOrderedSet couldn't be used as a method parameter of type NSMutableSet. So what happens if we make NSMutableOrderedSet a subclass of NSMutableSet as well?

// @interface NSOrderedSet : NSSet
// @interface NSMutableOrderedSet : NSMutableSet

NSOrderedSet* immutable = [NSOrderedSet orderedSet];
NSMutableOrderedSet* mutable = [immutable mutableCopy];

[mutable isKindOfClass:[NSSet class]]; // YES
[mutable isKindOfClass:[NSMutableSet class]]; // YES
[mutable isKindOfClass:[NSOrderedSet class]]; // NO (!)

This is perhaps even worse, as now NSMutableOrderedSet couldn't be used as a method parameter expecting an NSOrderedSet.

No matter how we approach it, we can't stack a mutable / immutable class pair on top of another existing mutable / immutable class pair. It just won't work in Objective-C.

Rather than subject ourselves to the perils of multiple inheritance, we could use Protocols to get us out of this pickle (as it does every other time the spectre of multiple inheritance is raised). Indeed, Foundation's collection classes could become more aspect-oriented by adding protocols:

  • NSArray : NSObject <NSOrderedCollection>
  • NSSet : NSObject <NSUniqueCollection>
  • NSOrderedSet : NSObject <NSOrderedCollection, NSUniqueCollection>

However, to reap any benefit from this arrangement, all of the existing APIs would have to be restructured to have parameters accept id <NSOrderedCollection> instead of NSArray. But the transition would be painful, and would likely open up a whole can of edge cases... which would mean that it would never be fully adopted... which would mean that there's less incentive to adopt this approach when defining your own APIs... which are less fun to write because there's now two incompatible ways to do something instead of one... which...

...wait, why would we use NSOrderedSet in the first place, anyway?

NSOrderedSet was introduced in iOS 5 & Mac OS X 10.7. The only APIs changed to add support for NSOrderedSet, though, were part of Core Data.

This was fantastic news for anyone using Core Data at the time, as it solved one of the long-standing annoyances of not having a way to arbitrarily order relationship collections. Previously, you'd have to add a position attribute, which would be re-calculated every time a collection was modified. There wasn't a built-in way to validate that your collection positions were unique or that the sequence didn't have any gaps.

In this way, NSOrderedSet is an answer to our prayers.

Unfortunately, its very existence in Foundation has created something between an attractive nuisance and a red herring for API designers.

Although it is perfectly suited to that one particular use case in Core Data, NSOrderedSet is probably not a great choice for the majority of APIs that could potentially use it. In cases where a simple collection of objects is passed as a parameter, a simple NSArray does the trick--even if there is an implicit understanding that you shouldn't have duplicate entries. This is even more the case when order matters for a collection parameter--just use NSArray (there should be code to deal with duplicates in the implementation anyway). If uniqueness does matter, or the semantics of sets makes sense for a particular method, NSSet has and remains a great choice.

So, as a general rule: NSOrderedSet is useful for intermediary and internal representations, but you probably shouldn't introduce it as a method parameters unless it's particularly well-suited to the semantics of the data model.

If nothing else, NSOrderedSet illuminates some of the fascinating implications of Foundation's use of the class cluster design pattern. In doing so, it allows us better understand the trade-off between simplicity and extensibility as we make these choices in our own application designs.

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